WELDING RESEARCH Fusion Zone Microstructure and Geometry in CompleteJointPenetration LaserArc Hybrid Welding of LowAlloy Steel A process map indicates a martensitefree microstructure can be maintained over a wide range of welding parameters BY H. L. WEI, J. J. BLECHER, T. A. PALMER, AND T. DEBROY APRIL 2015 / WELDING JOURNAL 135-s Introduction Hybrid laser-arc welding is a process that combines laser beam welding and conventional arc welding in order to incorporate the benefits of both processes (Refs. 1–4). Hybrid laser-gas metal arc (GMA) welding produces wider weld pools than autogenous laser welding, and deeper weld penetration than GMA welding with the same parameters (Refs. 5–8). The combination of laser and arc energy sources allows for complete jointpenetration welds to be achieved at significantly higher welding velocities in a single pass, while at the same time allowing large root openings in weld joints to be bridged (Refs. 9, 10). As a result, welding productivity can be greatly enhanced over that achieved by either laser or GMA welding alone (Refs. 11–13). In addition, hybrid laser- GMA welding has significant advantages in acquiring the desired weld metal microstructures, since lower cooling rates can be more easily obtained than in autogenous laser welding. However, martensite, which has very low ductility and toughness (Ref. 14), can still form in hybrid welding (Ref. 15). Previous work on the hybrid laser- GMA welding of steels has largely focused on the experimental postcharacterization of weld geometries, microstructures, and mechanical properties (Refs. 15–21). However, these postmortem evaluations provide little detail on the evolution of weld pool geometries and the cooling rates within the fusion zone. In order to understand and predict weld metal microstructural evolution, the thermal cycles experienced during these welding processes must be known. The interactions between the heat sources and materials during complete-jointpenetration hybrid laser-GMA welding lead to rapid thermal cycles in the weld pool, which in turn impacts microstructure evolution. Phase transformations during cooling in the weld fusion zone have been extensively investigated both experimentally and theoretically. Bhadeshia et al. developed a phase transformation model (Refs. 22–24) based on thermodynamics and phase transformation kinetics. This model can quantitatively predict the microstructures and properties of weld deposits for different alloy compositions, cooling rates, and prior austenite grain sizes. Direct measurement of temperature profiles in the interior of the weld pool still remains a major challenge. On the other hand, a well-tested three-dimensional mathematical model can provide accurate temperature fields and cooling rates at discrete locations throughout the fusion zone (Refs. 25, 26). Several studies focused on the numerical modeling of the fluid flow and heat transfer conditions within the molten weld pool of hybrid laser-arc welding (Refs. 6, 27). Ribic et al. (Ref. 27) numerically studied the effect of laser arc separation distance and laser power on heat transfer and fluid flow in partial-penetration hybrid laser-gas tungsten arc (GTA) welding by using a three-dimensional numerical model. They found that the distance between the laser and arc signifi- ABSTRACT The fusion zone geometry and microstructure in completejointpenetration hybrid laser gas metal arc welds of a lowalloy steel are examined experimentally and theoretically. Weld geometry and spatially variable cooling rates are investigated using a threedimensional heat transfer and fluid flow model. Experimentally measured microstructures are compared with those estimated from a microstructure model based on kinetics and thermodynamics of phase transformations, for a range of laser arc separation distances and heat inputs. Considerable variations in both cooling rates and microstructure were observed for the range of process parameters utilized. In fact, the experimental results and calculations show that for the same heat input, a predominantly ferritic and predominantly martensitic microstructure can be obtained, depending on the laser arc separation distance and resulting cooling rate. A process map is constructed showing the effect of welding speed, laser power, and laser arc separation distance on cooling rates and microconstituent volume fractions. The map indicates a martensitefree microstructure can be maintained over a wide range of welding parameters. KEYWORDS • Hybrid Welding • Heat Transfer and Fluid Flow • Microstructure • Complete Joint Penetration • Laser • Gas Metal Arc • Cooling Rate H. L. WEI (huw15@psu.edu), J. J. BLECHER, and T. DEBROY are with Department of Materials Science and Engineering, and T. A. PALMER is with Applied Research Laboratory, The Pennsylvania State University, University Park, Pa.
Welding Journal | April 2015
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